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Microbes took their breath away

By
JEFF HECHT in
BOSTON

The eight people who spent two years sealed inside Biosphere 2 began
to run short of breath because the scientists who created the ecosystem
in the Arizona desert made the artificial soil too rich. Decay of the organic
material in the soil consumed so much oxygen that eventually more had to
be pumped in.

Plants in the sealed biosphere should have produced enough oxygen to
offset consumption by animals and micro-organisms, while consuming exhaled
carbon dioxide. In the event, plant productivity was much lower than anticipated,
but that alone could not account for the oxygen deficiency. Scientists suspected
soil microorganisms were consuming far more oxygen than anticipated, but
were puzzled because they could not find the CO2 that the organisms
should have been producing.

Jeffrey Severinghaus of the Lamont-Doherty Earth Observatory in New
York state has now found that concrete inside the structure absorbed a massive
amount of CO2. Fresh concrete contains about 30 per cent calcium
hydroxide, which slowly combines with CO2 to form calcium carbonate
and water. Inside the biosphere, where CO2 levels were high,
the reaction proceeded very quickly. The extra fraction of carbonate in
the concrete roughly matches the missing volume of CO2.

The biosphere’s managers intend to paint the concrete to stop CO2
absorption – not because of its effect on the air, but because the same
reaction could cause steel reinforcing rods to rust and so weaken the structure.
The depletion of oxygen is harder to fix. It will take about 25 years to
reach an equilibrium between the decay of old organic matter in the soil
and production by plants, says Severinghaus.